Previous studies have indicated the presence of a macromolecular inhibitor of liver adenyl cyclase, but purification of this material has been thwarted by problems of instability. We have now identified a new macromolecular regulator or adenyl cyclase, generated by rod outer segment photoreceptor membranes upon illumination. It is responsible for a 25 fold difference in cyclase activity found between dark (220 pM cAMP/mg protein/10 min.) and illuminated (8pM cAMP/mg protein/10 min.) photoreceptor membranes. This inhibitor is a heat labile, non-dialysable factor which resists sedimentation at 60,000xg. It shows good storage properties (stable for more than a week at 5 degrees) and inhibits cyclase in other tissues as well, including liver and melanoma. This inhibitory material from rod outer segment membranes is not only important for understanding the molecular basis of photoreceptor function but also may provide a much needed entre into the problems surrounding the molecular transactions which control cyclase activity.